POTAMOPHILA 



teeth ; tlie third, " Dents later ales ecarttes^'' v/hereas, in 

 one valve we cannot find any lateral teeth, and in the 

 other they are very obscure, and placed near the cardinal 

 teeth ; the fourth, " Les impressions musculaires paraissent 

 double de chaque cote^^ whereas, upon careful examination, 

 we find no double appearance of the muscular impressions. 



An equivalve, transverse, subtriangular shell, of a 

 very thick substance, and covered on the outside with a 

 thickish olive green epidermis, which is generally worn off 

 at the urahones^ and the shell eroded, as in most other fresh 

 water bivalves. The cardinal teeth are thick and large : there 

 are two in the right-hand valve, which are united towards 

 the umbo ; three are observable in the other valve, placed 

 in the form of a triangle, the anterior of these is obliquely 

 elongated ; the intermediate one is thick at the base, and 

 pointed at the apex, it is the smallest, and is quite sepa- 

 rated from the other two, it fits into the cavity formed be- 

 tween the two united teeth in the right-hand valve; the 

 posterior tooth in the left-hand valve is the largest, it is 

 oblong, its apex is prominent, and there is a small cavity 

 near its base. There are very indistinct lateral teeth in the 

 right-hand valve, which are placed not far from the car- 

 dinal teeth; we cannot find any lateral teeth in the other 

 valve. The Cardinal ligament is external, short, promi- 

 nent, swollen, and the parts to which it is attached are also 

 prominent. There are two muscular impressions in each 

 valve, which are very deep towards the base of the shell. 



Only one species of this Genus is known ; it is the Venus 

 subviridis of Gmel. but is very distinct from Venus; beneath 

 the epidermis the shell is very white, sometimes violet co- 

 loured near the base, and marked with several violet rays. 

 Our engraving is taken from a specim^en in which the shell 

 is colourless; the epidermis alone having some rays of a 

 darker colour than the rest. It is a scarce shell, and highly 

 valued by Collectors. Lamarck says, it is an inhabitant of 

 rivers in the island of Ceylon, and the East Indies : our 

 specimen is from the river Congo ; several were brought 

 from thence, by the survivors of the late unfortunate 

 expedition. 



